Conveyor systems, in particular vane pumps, are already being employed for conveying sausage meat from hoppers into filling tubes by which sausage casings are filled with the sausage meat. Such vane pumps are already described, for example, in EP 0432388 B2 and EP 1040758 A1. Vane pumps consist of a pump housing and a rotor eccentrically arranged therein which can be caused to rotate. Radially movably mounted vanes are arranged in the rotor and form conveyor cells together with the wall of the pump housing, the bottom and the lid of the housing, and the outer surface of the rotor. The pump housing has a meat inlet and a meat outlet. The sausage meat can be pushed, for example, from the outlet into a filling tube.
When pasty masses are filled with a vacuum filler, a vacuum is generated in the conveyor system by means of a vacuum pump. This vacuum partially or quasi completely withdraws the gas/air content from the mass to be filled, which is normally desired. By this, the end product will be compact and non-porous.
However, there are also end products which are to keep their air content completely or partly. To achieve this, the degree of vacuum in the conveyor system is lowered. By this, no air or only a portion of the air content is withdrawn from the mass to be filled. The product keeps its initial consistency and is in this case not compacted in an undesired way. Examples of end products whose gas proportion should not or only partially be withdrawn, or which should not be compacted, are: precooked meat loaf, Bavarian veal sausage, caviar, all masses that should remain creamy as well as grainy products.
Up to now, the vacuum pump has been operated in nominal load operation. To reduce the degree of vacuum and adapt it to the end product, a certain amount of external air is supplied to the vacuum system with a manually or electrically operated valve. By this, the pressure difference between the vacuum system and the air proportion of the medium to be filled is reduced, so that the gas proportion in the medium to be filled is maintained completely or partially. The lower the applied vacuum is, the less grainy products are compacted.
Each positive-displacement pump, here the conveyor system of the vacuum filling machine, has an inlet side and a pressure side. Depending on the tightness of the pump, a more or less high vacuum is formed by the enlargement of the chambers of the conveyor system on the inlet side. Usually, the formation of a preferably constant vacuum is additionally supported on this side of the conveyor system by means of the vacuum pump, as described above. If now the degree of this generated vacuum, as illustrated above, is to be reduced, e.g. by the controlled supply of external air, the pressure on the inlet side of the conveyor system can be lower than the pressure in the vacuum system. Thereby, external air or fresh air supplied from outside is sucked from the vacuum system into the conveyor system.
In the process, the following problems have arisen. The filling medium can be contaminated by bacteria. The filling of the conveyor system chambers is not uniform, inevitably leading to increased weight fluctuations of the ejected portions. The vacuum pump must suck off an unnecessarily high amount of air, resulting in technical problems in some vacuum pump types. The vacuum pump is operated at an unnecessarily high power and thus energy is wasted.